The present invention relates to impulse radio type Ultra-Wideband (UWB) communication systems and, in particular, it concerns a method and device for generating a very short and high power bandpass pulse with low power consumption, with an inherent capability for binary phase shift keying (BPSK) modulation.
Ultra-wideband (UWB) communication is an attractive technology for personal and local wireless networks. Instead of transmitting and receiving continuous modulated sinusoidal waveforms as in carrier-based systems, impulse radio type UWB communication systems transmits short pulses with gaps of no energy between them. Such pulses are designed to occupy wide bandwidth, up to several GHz of spectrum.
For UWB systems occupying a large portion of the 3.1 GHz to 10.6 GHz band allowed by the Federal Communications Commission (FCC) rules a pulse width of 50-100 ps would be required. Other UWB systems using narrower spectrum, but still within the minimum bandwidth requirements of the FCC, would require longer pulses but still in duration of less than 1-2 ns. The wide bandwidth in turn allows a low power spectral density for a given transmission power, leading to the claim that it will not interfere with other users of that band.
Some (but not all) advantages of UWB technology are:
1. UWB provides a wide bandwidth signal more simply than other techniques.
2. Self interference in a UWB network is very low, allowing a large number of UWB terminals to operate in a given area.
3. The use of narrow pulses makes UWB very tolerant to multipath use.
4. UWB can provide accurate range information between a transmitter and a receiver, even down to a few centimeters.
In some UWB systems, like those described in patents application WO 2003/098528 entitled METHOD AND SYSTEM FOR DISTANCE DETERMINATION OF RF TAGS, the UWB pulses are organized in short bursts of relatively close pulses, i.e. about 10 ns between pulses, modulated with a Barker sequence.
The use of a burst of pulses allows a reduction in the pulse peak power, however relatively large pulses may still be required to achieve a decent power while at the same time conforming to the FCC UWB limits.
One technical difficulty to solve when designing UWB devices is how to generate high power pulses of short duration under the constraint of low power consumption from the power supply.
Prior art systems for generating pulses are known and there are many possible methods to generate UWB pulses. One method is a pulsed oscillator. In this method there is an oscillator with fast turn on and turn off times, and the output of this oscillator is the desired pulse, which can be further amplified or upconverted if necessary.
Traditional LC oscillator topology includes a reference current that creates high gain in the active device in order to obtain oscillation conditions. When these conditions are obtained, stable oscillations are generated. One method for generating a pulsed oscillator is describe in the article found at Webpage [http://www.eecs.berkeley.edu/Pubs/TechRpts/2006/EECS-2006-136.pdf]
In this method, turning on and off the reference current will turn the oscillation on and off such that a pulse is generated. A drawback of this method is that the turn on and off times of the oscillator are high, so wide bandwidth pulses necessary for UWB cannot be achieved. Fast turn on and off is achieved by a switch that shortcuts the inductor, and when the switch is released, the oscillation starts. When the switch is closed, the oscillations decay. In order to create fast start up of the oscillations, reset switches short the outputs temporarily to two different voltages (e.g. one to VDD and other to ground (GND)). This reset circuit should operate for a very short time, less than half a cycle, to avoid reducing the Q of the tank and disturbing the oscillation condition. Generating such short pulses (on the order of 100 ps) with the load of large switch transistors is a technological challenge.
In another method described in US Patent Application No. 2006/0039448 A1, a baseband pulse is generated by a ring oscillator topology and then the baseband pulse is upconverted by mixing with a continuously working oscillator generating the radio frequency (RF) frequency. This method has disadvantages of high power consumption and low output power.
In U.S. Pat. No. 3,649,918, a high Q cavity resonator is used in order to achieve oscillations in high energy. A short pulse is obtained by connecting the oscillator to the load by a switch for a short duration. The disadvantage of such approach is that the oscillator is consuming high power for a long duration, much longer than the needed pulse. In addition, a high Q resonator is required, which is not available in current Very-large-scale integration (VLSI) technology, so expensive external devices would be necessary.
Other methods to generate RE pulses do not rely on a pulsed oscillator. In U.S. Pat. No. 4,873,499, the pulse is generated by a step recovery diode (SRD) connected to the base of a transistor, which generates a fast rising electrical voltage step that is then converted into an impulse by one or more capacitors, which differentiate the step. The transistor is also driving a resonant circuit that generates the pulse. Design with a SRD diode suffers from variations in pulse position with temperature, and it also has a drawback of low output power.
Another requirement for a UWB pulser is an ability to control the pulse duration, hence its bandwidth. The operating frequency of systems is increasing due to technological improvements, so the need for narrower or “ultra fast” circuits is important.
Representative prior methods for controlling the pulse duration include U.S. Pat. No. 6,433,720, which describes the pulse duration being determined by the level of the control signal. This is achieved by level-activated switching elements, which can be transistors or diodes.
However, for VLSI implementation, using Gallium arsenide (GaAs) technology is much more expensive than utilizing Complimentary Metal Oxide Semiconductor (CMOS) technology.
Other pertinent prior systems include U.S. Pat. No. 5,274,271, which describes a narrow and high power pulse that is generated by a nonlinear transmission line with series inductors and variable capacitors coupled to ground made from reverse biased diodes. This method is implemented by discrete components and requires a sophisticated board design (because of high power level 100 kW) so it is more expensive than the device of the present invention. In addition the short pulses are generated by optimizing the length of the nonlinear transmission line. This is more complicated than the present invention. Also, the pulse repetition rate is limited mainly by the speed of the driver circuit.
In other systems, like U.S. Pat. No. 6,586,999, a low power pulse generator is amplified by a high power amplifier, but this power amplifier is turned on for about the duration of the transmitted pulse to save power consumption. Disadvantages of this method include using a high power amplifier that often demands a driver amplifier so additional current and chip size are needed. Further, there is no ability to control turning on the amplifier with the transmitted pulse. Therefore, the amplifier will open and close some time before and after the transmitted pulse which requires more current consumption. In the device of the present invention, there is an optional class C amplifier 103, therefore, there is no current consumption until there is a high signal level at the input.
There is therefore a need for a method and device for generating a very short and high power bandpass pulse with low power consumption.